Static resistant electric initiator



March 14, 1961 RAMER 2,974,590

STATIC RESISTANT ELECTRIC INITIATOR Filed Oct. 2, 1957 2| 21a Fm 2m 37m EDWARD L4 RAM ER INVENTOR.

AGENT 2,974,590 STATIC RESISTANT ELECTRIC Y "FOR Edward L. Rainer, Hockessin, DeL, assignor to Hercules Powder Company, Wiimiugton, DeL, a corporation of Delaware Filed Oct. 2, 1957, Ser. No. 687,717 Claims. (Cl. 102-28) This invention relates to electric initiators substantially free from susceptibility to static electricity. In one aspect, this invention relates to an electric blasting cap containing a conductive body assembly in combination with a dielectric coating on the leg Wires, within the conductive body, to provide the cap with high resistance to premature firing by static electricity. In another aspect, this invention relates to a method for initiating explosives under conditions providing for improved protection against random shots caused by static charges, or stray currents, to the initiator.

Electric blasting initiators, or blasting caps, comprise in general a metal shell, an ignition plug within the shell, closing the shell and supporting the leg, or pin Wires of the firing circuit in fixed spaced relationship. The leg wires extend from outside the shell into and through the ignition plug and are connected beyond the plug by a high resistance wire ordinarily referred to as the bridge wire. A composition ignitable by heat developed from passage of electric current through the bridge wire is disposed in operative contact with the bridge wire so as to be so ignited. The ignition charge is in operative contact with a detonatable base charge to cause detonation of same, upon ignition. Generally, a detonatable primer charge having a heat sensitivity higher than that of the base charge is disposed intermediate the ignition composition and base charge and is adapted to be detonated responsive to heat from the bridge wire and to thereupon cause detonation of the base charge. Delay type initiators contain a slow burning, or fuse, composition intermediate the ignition composition and the primer or base charge as the case may be, and are ignitable by the burning ignition composition. The delay fuse, upon completion of its burning initiates the primer to cause detonation of the same with subsequent detonation of the base charge, or is in direct operative contact with the base charge only, as the case may be. Various sealing means, particularly for Waterproofing the interior of the cap, are provided in the shell above the ignition plug, i.e., generally completely encompassing the lead wires extending from the ignition plug toward the power source.

Normally used ignition compositions, as above indicated, are highly heat sensitive and, accordingly, a discharge of relatively high static voltage quite often causes ignition of the said composition and firing of the initiator. The art is aware that such accidental firings can result from a direct discharge from a lead wire in the shell to the shell wall in the locus of the ignition composition. Of course, the caps can also be fired by direct discharge through the bridge wire via the lead wires. The initiator can also be fired by the passage of a portion of a static charge through the bridge wire when the discharge is from shunted lead wires to shell at a point other than through the explosive charge.

While the danger of premature firing due to static discharge is present in all types of electric initiators, the methods employed in seismographic prospecting have tended to make this type of initiator more susceptible to static electric charges than regular electric blasting caps and delay blasting caps. Until very recent years, commercial seismograph caps generally required a discharge in the order of 5,000 volts to fire the cap by direct discharge through the ignition composition and a discharge in the order of 12,000 volts to fire the caps by heating of the bridge wire when the charge was supplied by a 750 2,974,590 Patented Mar. 14, 1961 micrornicrofarad capacitor and the discharge was from shunted lead wires to shell. It has been established that voltages and capacities of this magnitude may be developed by a man under proper conditions and that much higher voltages and capacities may be generated in equipment employed by seismic prospectors such as drill rigs and trucks. Conditions favoring high voltage static generation, such as low humidity, high winds, sandstorms, and the like, are common in the localities where most seismic prospecting is done. Consequently, it will be seen that a definite danger of accidental firing of seismic caps is present under such adverse conditions. As a result of this danger, seismographic prospecting organizations and blasting cap manufacturers have endeavored to raise the static resistance of all electric initiators and especially that of the seismic-type blasting cap.

Various structures have been proposed wherein one or both of the bared lead wires are connected to the cap shell by a conductive material outside the locus of the ignition composition. However, it is difficult with such a structure to maintain a proper balance of conductivity that will allow a discharge from both wires to the shell and still have sufficient resistance for protection against the lower voltage currents (stray currents) which attend many commercial blasting operations. Additionally it has been found that in some instances the static resistance of this type of structure diminishes with storage.

Other structures, each involving a semi-conductive plug assembly separated from the shell wall by a peripherally disposed air space or dielectric material, are disclosed and claimed in US. Patents 2,658,451 and 2,802,- 422, issued November 10, 1953 and August 13, 1957, respectively, to Charles F. Horne, and in US. Patent 2,802,421, issued August 13, 1957 to Charles F. Home and Edward L. Rarner, the said patents being assigned to Hercules Powder Company. However, somewhat time consuming steps are involved in the manufacture of such assemblies, particularly with reference to effecting proper placement, or centering, of the semi-conductive plug.

This invention is concerned with an improved initiator structure which is highly resistant to premature firing by high voltage static discharge from lead wire to shell and at the same time is highly resistant to premature firing by stray currents, and which can be fabricated in accordance with short-time-requin'ng manufacturing steps.

An object of this invention is to provide electric blasting initiators having high resistance to premature firing by static discharge and by stray currents. Another object is to provide electric blasting caps containing a new conductive body assembly for imparting high static resistance to the cap. Another object is to provide a meth od for the electric initiation of blasting charges under conditions providing for protection against random shots caused by static charges to the initiator. Other objects and aspects will be apparent to one skilled in the art in light of the accompanying disclosure and the appended claims.

In accordance with the invention there is provided an improvement in an electric blasting cap which comprises a conductive body disposed within the cap shell in direct contact with a section of each of the leg Wires within said shell and with the wall of said shell; insulation around all of each of said leg wire sections to insulate same from said conductive body; and as at least a portion of said insulation, a coating of a dielectric material having a thickness in the range of 0.0001 to 0.005 inch.

Further in accordance with the invention there is pro vided an improvement in the utilization of an electric blasting cap in the initiation of a blasting charge so as to protect against random initiation and detonation by static charges to the blasting cap, which comprises supporting a conductive material in direct contact with a section of each of the leg wires within' the said shell and with the Wall'of said shell; maintaining the entire portion of each said leg wire section insulated from said conductive body; maintaining as at least a part of the resulting insulation, a coating of a dielectric material having a thickness within the range of 0.0001 to 0.005 inch, on each of said wire sections; positioning said blasting cap in operative relation with a detonatable explosive charge so as to cause detonation of said charge, when fired; and then firing said blasting cap.

The invention is illustrated with reference to the drawings of which Figure 1 is a front sectional view of an electric blasting cap of the invention wherein a conductive plug assembly is disposed above a sulfur ignition plug of the sleeved type for supporting the ignition composition; Figure laby way of illustration in large detail shows an embodiment of the conductive assembly of the invention; Figure 2 is a view of the cap assembly of Figure '1 except that the sulfur ignition plug is of the cavity type; Figure 3 is a front sectional view of a cap assembly of the invention wherein the conductive assembly-functions as the ignition plug in lieu of a separate sulfur plug; Figure 4 is a view of the assembly of Figure 3 except that a matchhead-type ignition composition is utilized in lieu of a powdered, or a buttered, composition; Figure 5 is a view of an assembly the same as Figure 4 except that the ignition composition is a powder and the semi-conductive plug, as the ignition plug, is of the noncavity type; Figures 6 and 6a are detailed showings of an embodiment of a conductor assembly of the invention utilizing a dielectric member as a support for a coated conductive leg wire strip or surface within the conductor; and Figure 7 illustrates an embodiment of the conductor assembly in which the conductor is in direct contact with the shell wall along only a portion of its perimeter so as to only partially close the cap shell.

With reference to Figure 1, base charge 11, in shell 9 of electric blasting cap 10, is a suitable detonatable material such as pentaerythritol tetranitrate (PETN). Above and adjacent base charge 11 is a suitable primer charge 12 such as diazodinitrophenol positioned tocause detonation of base charge 11 when heatedby ignition of ignitable composition 13. Composition 13 can be a loose powder such as lead-selenium, or a paste buttered in the cavity formed by-paper sleeves 14 such as lead mononitroresorcinate/potassium chlorate. Ignition plug 16, such as formed from molten sulfur is disposed above composition 13 and, together with paper sleeves 14, forms cavity a. Semi-conductive plug 17 such as formed from a wax containing aluminum particles, e.g., 35-65 wax/Al weight basis, is disposed above plug 16 around leg wires 21 as a complete closure for shell 9. Suitable waterproofing and sealing plugs 18 and 19 respectively are disposed above plug 17 to also close the shell. Plug 18 is generally an asphaltic composition and plug 19 is generally a seal of sulfur or a plastic material as, for example, ethyl cellulose.

Leg wires 21 extend into shell 9 through plugs 19, 18, 17 and 16 and terminate in cavity a wherein they are connected by bridge wire 22, for example, a 0.002 inch diameter gold-plated nichrome wire, which upon being heated by passage of electric current therethrough initiates ignition of composition 13.

Leg wires 21 can be insulated by conventional insulation means 21a, for example a treated fabric or a plastic material such as ethyl cellulose, extending to a point within shell 9 into and through a part or all of plugs 18 and 19 but in no event through more than a portion of semi-conductive plug 17. Preferably insulation 21a covers that portion of the leg wire extending into and through plug 19 and into only a part of plug 18.

Dielectric coating 23 is disposed around each of the leg wire sections extending through semi-conductive plug 17 and is at least 0.0001 inchthick. The thickness of at least a portion of each coating 23 does not exceed 0.005 inch. The leg wire portions extending within plug 16 need not be coated but it is generally preferred from the manufacturing standpoint to extend the coating at least into a portion of plug 16 to assure during the manufacturing step that the contemplated leg wire section, or portion thereof, within the semi-conductive plug is coated. The wire extending toward the bridge wire from the end of coating 23 is generally bare inasmuch as there must be a bare portion to facilitate welding or soldering of bridge wire 22 to the terminal ends 21b of wires 21.

The now-preferred minimum thickness of the coating of the invention is about 0.0004 inch, the now-preferred value for the above said maximum thickness being 0.003

inch. 7 I

With reference to Figure 2 there is shown blasting cap 10a which 'is the same as that of Figure 1 except that in lieu of cavity a being formed by paper sleeves 14 and the bottom side of plug 16, the said cavity is formed by the bottom surface 24 of a suitable plug-16' such as sulfur. Ignition composition 13 of Figure 2 can be the same as that of Figure 1, preferably a buttered-in-paste composition.

With reference to Flgure 3 there is shown a cap as- I sembly 10b which differs from caps 10 and 10a of Figures 1 and 2 with reference to semi-conductive plugs 17 and 17'. In Figure 3 the semi-conductive plug 17 serves also as theignition plug (with associated sleeve structure) so that cap 10b is'free from a separate ignition plug 16 or 16' of Figures 1 and 2.

The assembly of Figure 4 differs from that of Figure 3 in that an ignition composition 13 of the matchhead type is utilized, cap assemblies 10b and 10c being otherwise the same.

With reference to Figure 5 there is illustrated the semi-conductive assembly 17" disposed directly on powdered ignition composition 13 to provide an assembly of the non-cavity type.

With reference to Figure 6 there is shown an embodiment of the conductive assembly of the invention in accordance with which, to opposite sides of a dielectric material 31, such as a phenolic resin-impregnated paper member, are secured conductive strips or surfaces 32 and 33, each of which serves as a leg wire section in the assembly and is connected at one end with the bridge wire 22 and at the other end with leads to the power source. Each of leg wire conductive elements 32 and 33 within semi-conductive plug 17" is coated with a dielectric coating material 23' in accordance with the invention. Conductor elements 32 and 33 can be in form of wire, metal foil, conductive resin, a painted on" surface such as a conductive suspension of silver particles, or the like. Any suitable dielectric element 31 can be utilized. The assembly of Figure 6 is, preferably, assembled as a core element of a conductive plug and then inserted as such a plug unit into the cap shell. Figure 6a is a view of the assembly of Figure 6 along the line 6a-6a of Figure 6 and is further illustrative of that assembly as a unit of a complete blasting cap. a a With reference to Figure 7 is illustrated a conductive assembly of the invention wherein the conductive body is in contact along only a portion of its perimeter with the shell wall. Thus conductive body 17a,'as shown, directly contacts both'leg'wires 21, coated with dielectric coating 23 of the invention, and by direct contact of radially extending portions 17b and withthe shell wall functions in exactly the same manner as plug 1717' of Figures 1-6.

Because the breakdovm voltage of the conductive body must be quite low in order'to give the desirable protection against static discharge, the initiator would have undesirably lo'w voltage breakdown characteristics inasmuch as the semi-conductive body directly touches the shell wall. However, due to dielectric coating 23 on all of the leg wire portions extending through the conductive plug, the voltage breakdown of the cap is greatly EXAMPLE 1 A number of seismic type caps similar to those of the drawings were prepared and tested for static resistance. The tests were made with a static charge supplied by 21 increased and offers protection against the type of stray 5 750 mmf capacitor discharged through the cap. Cam fiurrenbwhlch reasonably expected to 136 found motions were made with the externally shunted lead 111 blasting operatlcnsr I wires and the shell. The number of caps tested, the Voltage breakdown ranging up to 1000 V013, and 111 number of those shot (S) and the number that failed to some instances higher, is obtained employing the coated shoot (F) were noted. Data summarizing the results 1O leg wire-conductive body assembly of this invention of the tests are set forth in the following tabulation:

Table I Test Voltage Coating on Each Leg Coating Cap Tested Wire Within Serniand 11,000 16,000 21,000 30,000, Conduetwe Plug Thickness F F 9F1s 9F1S 5E 5F 10F 10F F 20F 10F 10F as base charge; diazodinitropheuol/KC10 as primer; lead Al/wax. 05/35,serniconductive plug surrounding and covering coated leg wires and disposed above a paper sleeved sulfur plug, as illustrated with reference to Figure 1; 0.002 gold-plated nichronie bridge wire; #22 copper lead wire.

ame as A except that in lieu of the semi-conductive plug per se, a conductive vinyl polymer was applied to the coated wire at the top of the sulfur plug and continuously down over the top of the sulfur plug and on to its sides wall.

in direct contact with the copper shell inner 0. Same as A except that a conductive rubber plug comprising rubber and magnetic carbon black was utilized in lieu of the Al/wax semi-conductive plug.

Same as A except that the leg wires extending through the semi-conductive plug were bare (no coating).

Same as A except that the sulfur plug was of the cavity type. FF saFmelag B except that the sulfur plug was of the cavity type. 1 .ai o

S=Shot. along with high resistance to premature firing by static discharge, when the thickness of the dielectric coating on the leg wire is as above described. On the other hand, extremely low voltage breakdown is obtained employing prior art cap assemblies wherein the conductive plug touches the shell wall and the leg wires within the conductive plug are bare. When the thickness is less than the 0.0001 inch minimum, the voltage breakdown is unduly low. When the said maximum thickness value, i.e., for the entire coating, is exceeded, the voltage breakdown is undmirably high with concomitantly undue lowering of the resistance to static discharge.

The invention is further illustrated with reference to the following examples:

The data of the foregoing tabulation demonstrate the high resistance of the cap assembly of the invention to premature firing by static discharge, and with especial reference to cap D, show that the dielectric coating does not unduly impair the function of the semi-conductive plug to direct high voltage static charges from the leg wire to the shell wall.

EXAMPLE 2 Test Voltage 0 iiif ap 1c Tested 220 Leg Wire Coating Hess, Inch Minutes Results Seconds Results 3 10F Polyvinyl acetal 0. 0005 3 5S None. A commercially available cap containing semi-conductive plug assembly. 3 5S None. (wax/Al semi-conductive plug only.)- 3 5F 30 4F1S Polyvinyl Acetal (h 0.0013 3 5F 30 5F 0. 0026 3 518 30 5F 0. 0006 3 5F 30 5F 0.0010 3 5F 30 4F18 O. 001 3 5F 30 5F 0. 0014 3 SF 30 4F1S d0 0. 0014 E. See Table I. for definition.

H. Commercially available electric blasting cap containing a conductive rubber plug (rubber, graphite) with no coating (bare) on the leg wires extending through it, and surrounded by a dielectric rubber slitted sheath in contact with the shell wall.

. Same as cap A of Table I, exceptior no coating on the leg wires extending through the semi-conductive plug. 2

. Same as #1 except #26 plain copper lead wire, and coating.

. Same as #4 except for coating.

. Same as #2 except for coating.

. Same as #0 except for coating.

. Same las #9 except for coating.

7 EXAMPLE 3 Gap Tested Number 10 NBD.

15,25, 60, 70, 70, 75, 75, 125, 140, 175 (Control,

no coating).

200, 200, 500, 500, 500, 600, 800, 900, 2NBD.

200, 200, 200, 400, 500, 5NBD.

NBD.

600, 600, 900, 7NBD.

10 NBD.

500, QNlBD.

500, QNBD.

*NBD No' breakdown. Caps A and O are the same as those respectively of Table I. Caps 1, 2, 3, 4, 5, 6, 7 and 8 are the same as those respectively of Table 1a. Same as #1 except for polyvinyl acetyl coating 0.005 inch thick.

EXAMPLE 4 A series of caps the same as cap A of Table 1 except for the specific coating indicated were tested for static susceptibility in accordance with the procedure of Example l. The data are summarized in the following tabulation:

8 lilla, montan, and carnauba and synthetic waxes such as those manufactured under the trade names Ceramid, Acrawax C, Flexo Wax and Bareco Wax. Various mixtures of these waxes may also be employed. Of this group Acrawax is preferred.

Instead of wax, however, rubber or rubber-like materials, resinous materials, sulfur and equivalent materials may be employed. Since it is desirable that the body of semi-conductive material be molded about the lead wires, readily moldable nonconductive materials are preferred.

The amount of a particular conductive filler employed in the semi-conductive body will depend on the conductivity of the particular material and on the degree of static protection desired. The method of forming the semi-conductive body about the lead wires will also dictate the optimum quantity of particulate conductive filler employed. In the preferred composition of wax and aluminum, it has been found that best results are obtainable when between 60 and 70% of particulate aluf minumis employed. The upper limit of 70% is primarily dictated by the fact that it is desired to employ a pourable mixture for molding. The use of more than 70% of aluminum powder usually results in a mixture which is undesirably viscous. It has been found that adequate protection against static discharge from the lead wire to the shell through the ignition composition can be obtained with about 60% of particulate aluminum. Lesser amounts of aluminum can be employed with very beneficial results. From all considerations a :65 mixture of Acrawaxzalu- 1 Caps 1a, 2, 4, 5, 6, 7 and 8 are respectively those of Table III.

R A polyamide.

S=Shot.

The examples, with reference to Tables I, III and IV illustrate that the high degree of static resistance of the initiator in accordance with the invention is obtained without sacrificing to an undesirable extent the equally essential high voltage breakdown characteristics.

Although it is usually preferred to employ a conductive metal powder as the conductive component of the semiconductive body, especially aluminum powder, other particulate conductive materials, such as conductive carbon may be employed.

The nonconductive component which acts as. abinder,

and carrier for the conductive particles is preferably a wax. The wax employed will desirably have a meltingv point of 120 F. or above in order to prevent substantial cold flow of the wax subsequent to manufacture. The melting point of the wax, or any other operable carrier, will, of course, depend largely upon practical considerations. For instance, if the waterproofing material employed is an'asphaltic composition which is poured. into the cap in molten form, a wax will be employed which has a melting point substantially higher than the temperature of the filling composition as it is poured into the shell. On the other hand, if a rubber or resin sealing plug is employed and is placed in position in cold form, the only consideration then is to use a wax which will not cold flow.

. it is assembled into the cap unit, it is within the scope of If a molten resin is employed, the same considerations then pertain as in the case of a molten j asphaltic composition.

Preferred waxes include such natural waxes as candethe invention to utilize any suitable conductor body as for example a cast metal or one made by impact extrusion of a metal foil. Thus the conductive body can be, for example, iron, brass, copper, zinc, or an alloy of such metals, cast or impact extruded foil as desired.

Although it is preferred that the dielectric coating of the invention be characterized by a thickness uniformly within the range of 0.0001 to 0.005 inch, it is sufilcient that the coating have the minimum thickness hereinabove described, i.e., of about 0.0001 inch and that at least about A; to of the coating surface have a thickness not exceeding the maximum value, i.e., not greater than about 0.005 inch.

Generally the coating surrounds the leg wire, i.e., is in direct contact with the given section of the leg wire along its entire area. However, as illustrated, with reference to Figure 6, the coating need only cover a portion of the given length of the leg wire so long as the remaining portion is insulated from the conductive body. Also,

it may be advantageous in some instances to extend a legv wire portion, covered with conventional insulation, a

such that the conventional insulation does not extend more than about M; to of the distance through the length of the conductive body. In such embodiments the coating is around, and in direct contact with the leg wire from the point of termination of the conventional insulation.

In accordance with one embodiment of the invention, one or both the lead wires within the conductive plug is characterized by a rough surface which provides a series of peaks and valleys on the wire so that when the coating material is applied it fills the hollows and covers the peaks to provide relatively thin areas of coating, thus producing high static resistance by directing the high voltage current to the peak of relatively low thickness.

It is an advantage of this invention that an electric blasting initiator which utilizes a semi-conductive plug assembly andalso oifers high resistance to premature firing by static discharge and high voltage currents can be manufactured without requiring the time consuming steps for centering the conductive plug so as to isolate it from the shell wall as is required in the manufacture of the assemblies referred to herein with reference to the hereinabove referred to U.S. Patents 2,658,451, 2,802,421 and 2,802,422. Indeed, no more is required in the preparation of the initiator of the invention than is called for in the preparation of electric blasting caps of the simplest design.

The coating, in accordance with the invention, is applied to both leg wire sections within the conductive plug in order to assure the necessary resistance against static discharge and firing by stray currents.

Generally the minimum firing current of the initiator is undesirably raised if the diameter of the bridge wire exceeds about 0.0015 inch and it is desired to employ,

generally, a bridge wire having a diameter of from about 0.0015 to 0.0030 inch, 0.002-00025 being generally the preferred. Preferred bridge alloys are those of nickel and chromium such as manufactured under the trade names of Tophet C, Ohmax and Jelliif 1,000. Platinum alloys can be advantageously employed when desired.

The cap shells are preferably of any conductive metal such as brass, copper, ferrous metals and various alloys, although plastic cap shells are within the scope of the invention. The lead and leg wires may be made of any of the conventional materials such as copper, iron, and the like. Lead wires are often advantageously tinned. The lead wires may be insulated with any desired material such as cotton servings, rubber or various plastics.

The base charges may be formed from any secondary detonative explosive such as pentaerythritol tetranitrate, cyclonite, tetryl, trinitrotoluene, and the like, and may be cast or pelleted as well as pressed when the nature of the explosive permits. When a priming charge is employed, any primary explosive or mixture can be used such as diazodinitrophenol, diazodinitrophenol-potassium chlorate, lead azide, 'lead styphnate, and mercury fulminate. Any of the known ignition compositions may also be employed such as finely divided diazodinitrophenol-chlorate, lead mononitroresorcinate-chlorate, fulminates, leador tin-selenium mixtures and the like.

The cap shell may be sealed by more than one sealing layer as shown in the drawings or a single seal may be employed such as a rubber or asphaltic sealing plug or a cast or molded resin plug, and the shell suitably crimped.

Exemplary dielectric coating materials of the invention in addition to those already referred to are cellulose acetate, ethyl cellulose, vinyl chloride, vinyl chloride acetate copolymer, vinylidene chloride, vinyl acetate copolymer, polystyrene, methyl methacrylate, polyethylene, and tetrafluoroethylene.

As will be evident to those skilled in the art, various modifications can be made or followed, in the light of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or from the scope of the claims.

What I claim and desire to protect by Letters Patent is:

1. A static resistant initiator comprising in combination an elongated metallic shell, closed at one end; an electrically conductive body, within said shell, spaced from the closed end thereof transverse to the shell axis and in contact along at least a portion of its periphery with the inner wall of said shell; a pair of electric conductor wires extending into said shell through the end thereof opposite the said closed end and through the said conductive body and terminating intermediate said conductive body and the said closed end of the shell; a bridge wire, within said shell, intermediate said conductive body and said closed end, and connecting the terminal ends of said pair of conductor wires; an ignition composition, within said shell, intermediate said conductive body and said closed end and in direct contact with said'bridge wire; a layer of a dielectric material along, and completely around, the entire length of each said conductor wire within the said conductive body in direct contact with said wire and with said conductive body; the said dielectric layer being characterized by a minimum thickness of 0.0001 inch and by a thickness along at least a portion of each said length of conductive wire not greater than 0.005 inch; a dielectric body within said shell transversely disposed across a por tion thereof intermediate said conductive body and the end of said shell opposite said closed end, and said dielectric body closing said shell.

2. A blasting cap assembly of claim 1 wherein said conductive body is in direct contact along its entire periphery with the inner wall of said shell.

3. An initiator of claim 1 wherein said conductive body is semi-conductive and is a wax plug containing metal particles suspended therein.

4. An initiator of claim 1 wherein each said layer of dielectric material within said conductive body and char acterized by the said thickness not exceeding 0.005 inch extends along at least A3 of the length of the said wire which it surrounds.

5. An initiator of claim 1 wherein at least a portion of the surface of each said wire, along the length thereof within said conductive plug, is irregular in form of peaks and valleys.

6. An initiator of claim 1 wherein the said dielectric layer is characterized by a minimum thickness of 0.0004 inch and by a thickness along at least a portion of the said wire length not greater than 0.003 inch.

7. An initiator of claim 1 wherein each said layer of dielectric material is a coating of a resinous material selected from the group consisting of a polyvinyl acetal, a polyamide-polyvinyl formal and a polyurethane.

8. An initiator of claim 1 wherein said conductive body is in contact only at a plurality of spaced apart points along its periphery with the said inner wall of said shell.

9. A static resistant initiator comprising in combination an elongated metallic shell, closed at one end; an electrically conductive body, within said shell, spaced from the closed end thereof transverse to the shell axis and in contact along at least a portion of its periphery with the inner wall of said shell; a dielectric body within said shell transversely disposed across a portion thereof intermediate said conductive body and the end of said shell opposite said closed end, and closing said shell; a pair of electric conductor wires extending from the outside of said shell into said shell through the end thereof opposite the said closed end and through the said dielectric and conductive bodies and terminating intermediate said conductive body and the said closed end of the shell; a bridge wire within said shell intermediate said conductive body and said closed end, and connecting the terminal ends of said pair of conductor wires; an ignition composition within said shell intermediate said conductive body and said closed end and in direct contact with said bridge wire; electrical insulation around at least a portion of each said conductor wire leading to said dielectric body and around the entire portion of said conductor wire within said conductive body; and at least a portion of the length of said insulation around each said conductor wire within said conductive body being a'coating of a dieleo, tric material characterized by a thickness less than that of the remainder of said insulation and within the range of from 0.0001 to 0.005 inch.

10. A static resistant initiator comprising in combination an elongated metallic shell, closed at one end; an electrically conductive body, within said shell, spaced from the closed end thereof transverse to the shell axis and in contact along at least a portion of its periphery with the inner wall of said shell; an elongated body of a dielectric material within said shell and longitudinally extending through said conductive body; a pair of electrical conductor wires extending into said shell through the end thereof opposite the said closed end and through the said conductive body and terminating intermediate said conductive body and the said closed end of the shell; a bridgewire within said shell intermediate said conductive body and said closed end, and connecting the terminal ends of said pair of conductor wires; the said conductor wires, along their entire length within said conductive body, being supported apart and in direct contact with a side of said elongated body of dielectric material; an ignition composition, within said shell, intermediate said conductive body and said closed end and in direct contact with said bridge wire; and a coating of dielectric material, within said conductive body, in direct contact therewith and also, in direct contact with the entire p ortion of each said'wire length out'ofcontact with said elongated dielectric body; each said coating of di electric material being characterized by a minimum thickness of 0.0001 inch and by a thickness'along at least a portion of each said length of conductor wire within said conductive body not greater than 0.005 inch; and a dielectric body within said shell transversely disposed across a portion thereof intermediate said conductive body and the end of said shell opposite said closed end, and the last said dielectric body closing said shell.

References Cited in the file of this patent UNITED STA'IES PATENTS 2,086,548 Handforth July 13, 1937 2,237,932 Handforth Apr. 8, 1941 2,477,458 Johnson July 26, 1949 2,658,451 Horne Nov. 10, 1953 2,741,179 Taylor et al. Apr. 10, 1956 2,802,422 Home Aug. 13, 1957 2,882,820 Young Apr. 21, 1959 FOREIGN PATENTS 430,750 Great Britain June 20, 1935 

